1. The Field of the Invention
The present invention relates generally to the field of articulating prosthetic joints for use in a human or animal body. More particularly, it concerns a socket member for use in a ball and socket prosthetic joint device such as a hip stem prosthesis.
2. The Background Art
It is known in the art to replace a joint, such as the hip joint, with an artificial hip stem replacement. Numerous artificial implants are available which can be installed to replace the natural hip joint with an artificial ball and socket combination. A passage called the medullary canal is reamed or bored in the upper end of the femur. A stem or femoral component of an artificial implant is inserted into the reamed portion of the medullary canal in a secure, seated position. A neck member extends outward and away from the stem and terminates in a spherical knob or ball for insertion into a socket member secured within the acetabulum of the hip. The ball and socket member are maintained in rotational contact with one another about the three major orthogonal axes.
Prosthetic joint implants are thus generally constructed with two bearing elements maintained in slidable contact. Such prosthetic joint implants are typically constructed of biocompatible metals, such as stainless steel (e.g., 316LC), cobalt alloys (e.g., ASTM F-75) and titanium alloys (e.g., 90% Ti, 6% Al and 4% V), ceramics (e.g., aluminum oxide, Al.sub.2 O.sub.3 and zirconium oxide, ZrO.sub.2), and biocompatible polymers such as ultra-high molecular weight polyethylene (UHMWPE), polyetheretherketone (PEEK), and medical grade polysulfone. Recognizing that the present invention is applicable to other joints as well, total hip replacement (THR) prostheses are discussed herein for purposes of example.
In one THR implant, a metal ball, or head, is fixed to the patient's femur by a hip stem and a UHMWPE cup is fixed to the patient's acetabulum. The metal head articulates or rides in the UHMWPE cup. Generally, to reduce friction and wear, the bearing surface of the metal head is polished and that of the UHMWPE cup is smooth. In addition, these bearing surfaces are often lubricated with synovial fluid while in the patient's body. Even so, when the polished ball articulates in the UHMWPE cup, some wear of the UHMWPE takes place. This wear generates debris in the form of submicroscopic UHMWPE particles.
Scientific studies using joint simulators have shown that the wear process in ball and socket prostheses generates hundreds of thousands of polyethylene particles with each step the patient takes. Retained lubricant from simulated hip joint testing machines have revealed literally billions of microscopic particles of the polymer cup (in simulating a one year wear cycle in a human hip). These particles tend to speed up the wear process.
Scientific studies throughout the world have shown that such high quantities of wear debris can generate a bone disease called osteolysis in patients who have had a total hip replacement prosthesis. A high incidence of osteolysis (nearly 21%) is being discovered in prosthesis implanted after five to seven years. Osteolysis destroys healthy bone but seldom causes pain, sometimes causing a bone to become susceptible to fracture before the patient knows anything is wrong. In most cases, debris resulting from wear between the metal ball and polymer cup surfaces is named as the primary cause of the disorder. Biological analysis of such damaged bone sections have established this disorder as being one of the most critical orthopaedic problems today.
Point contacts and resulting nonuniform load transfer between the ball and socket components further contribute to wear debris generation. As the contact area between the bearing surfaces decreases, the stress that is transmitted between the surfaces increases. The increased stress not only increases wear debris generation, it can also raise the possibility of fracture of one or both surfaces, especially when the stress concentration becomes highly localized.
There are other sources of wear debris in joint prostheses besides the ball and cup interface. The socket portion of many THR implants includes a two-piece cup system, with the UHMWPE cup seated in a metal shell. It has been reported that large quantities of polyethylene debris are generated between the UHMWPE cup and the metal shell. The fit between the cup and shell is typically close but not perfect. Normal walking is likely to produce sufficient movement between the cup and the shell to generate such debris. The metal shell is usually fastened directly to the patient's acetabulum bone, often with screws. The generated debris may gradually work its way out of the space between the polyethylene cup and metal shell. The debris can then migrate through the screw holes or other gaps between the metal shell and the surrounding acetabulum bone.
Prior attempts at limiting movement between the cup and shell have included forming a plurality of serrations or scallops on the inside rim of the metal shell. The cup and shell are then locked together by snap-fitting the cup into the shell, with the metal serrations or scallops locking into the outside rim of the polymeric cup. While this arrangement provides initial macro-dimensional stability between the cup and shell, the cup begins to move microscopically within the shell with time and use because of slight dimensional differences between the congruent surfaces.